15 facts about our planet for Earth Day

It's easy to take Earth for granted, since we see it every day. It becomes — it is — part of life's background.

But when you see the world through the eyes of science, nothing is mundane. We live on the surface of this great giant space-borne water-laden spinning rock, separated from the rest of the Universe beneath a thin veil of nitrogen and oxygen molecules. Even though you're immersed in its influence, what do you really know about the Earth?

2) The Earth has a volume of about one trillion cubic kilometers. Can you picture a cube 1000 meters high, 1000 meters deep, 1000 meters across? Now picture a trillion of them. That's the Earth.

Actually, if you were that big, it would be easy.

3) The Earth has a mass of 6,000,000,000,000,000,000,000,000 kilograms, or, if you prefer, 6 sextillion tons. In pounds, that's actually . . . 0. Nothing. Mass is a measure of how much stuff an object contains, but weight is how hard gravity pulls on that mass. The Earth is in space, orbiting the Sun, so it's in freefall. It has mass, but no weight at all.

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4) The Earth isn't a perfect sphere. It spins, so it's a flattened at the poles a little bit. The diameter through the poles is 12,713.6 kilometers (7882.4 miles), but it's 12,756.2 kilometers (7908.8 miles) through the equator. That difference of 43 kilometers is only about 0.3 percent, though, so really we're pretty close to a perfect sphere.

5) Not only is it flattened, but the gravitational forces of the sun and moon (what we call tides) distort its shape even more, pulling bulges out from it. The Earth is lumpy! Out in the deep ocean, the bulge of water due to the sun and moon can have an amplitude (change in height from minimum to maximum) of about a meter (40 inches). The solid Earth deforms due to the tides, too, with an amplitude of roughly 50 centimeters (20 inches). Even the air is affected by tides; though there are several factors that greatly complicate it (like expansion due to heating from the sun during the day, and, simply, weather).

6) There is no physical place where Earth's atmosphere stops and space begins; the air just gets thinner and thinner and eventually fades away. But we love definitions, so the official height above the Earth's surface considered to be where space begins — called the KÃ¡rmÃ¡n line — is at an altitude of 100 km. Anyone who gets higher than that is considered an astronaut.

7) The Moon's radius is about 1/4 that of the Earth's, making it the largest satellite compared to its parent planet. Charon, Pluto's biggest moon, is about half the diameter of Pluto itself. So if you don't consider Pluto a planet, the Earth and Moon win.

8) The Moon is farther away from Earth than you think. As an analogy, if the Earth were a basketball, the Moon would be the size of a tennis ball 7.4 meters (24 feet) away.

9) The Earth's atmosphere is only transparent to a narrow slice of the electromagnetic spectrum. What we call visible light (mostly!) gets through, but most flavors of infrared, ultraviolet, X-rays and gamma-rays are stopped cold. Those last few are dangerous to life as we know it, so that works out well. But it's not a coincidence: if our air didn't protect us, we'd have evolved differently.

12) An asteroid, 2010 TK7, shares an orbit with the Earth. It's about 300 meters (1000 feet) across, and never gets close enough to us to be a danger.

13) The Earth orbits the Sun on an ellipse. The shape changes slightly over time due to the influence of the other planets, but on average the closest we get to the Sun (perihelion) is about 147.1 million kilometers (91.3 million miles) and the farthest (aphelion) about 152.1 million kilometers (94.3 million miles). That difference is only about 3 percent, which by eye is very nearly a perfect circle.

15) The Earth's atmosphere weighs 5 quintillion kilograms, or 5000 trillion tons! You can do this math yourself: Weight is equal to pressure times area. Atmospheric pressure on the Earth's surface is about 1 kilogram per square centimeter. Multiply that by the number of square centimeters on the Earth's surface and you get the weight of all that air. Hint: The area of a sphere is 4 x pie x radius squared. [Note: Yes, I know kilograms are a mass and not a weight.]

And a bonus, because it's important:

16) The Earth is the only place in the entire Universe where we know that life exists. But that won't be true forever.

To us, our Earth seems huge, solid, tailor-made for us, and permanent. But that is just one perspective, born of living on its surface. From a different perspective, none of those things is true. Seen from space, it looks much less unbreakable. Seen from deep space it shrinks to nothing more than a dot, barely visible in the reflected light of the Sun. From another star, even seeing our planet at all would be a colossal task. We are, after a monumental effort spanning decades, only just now finding other planets orbiting other stars.

Is any like Earth? Almost certainly, and in fact there may be billions of planets like ours orbiting alien stars. But while they are like ours, they aren't ours. As with any individual, our world is unique, and precious, and wonderful. Let's keep it that way.

Phil Plait, the creator of Bad Astronomy, is author of "Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing 'Hoax' " and "Death from the Skies! These Are the Ways the Universe Will End."

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